Genetics of alcoholism and substance abuse

The Role of Epigenetics in Addiction: Clinical Overview and Recent Updates

Addiction is an international public health problem. It is a polygenic disorder best understood by accounting for the interplay between genetic and environmental factors. A recent way of perceiving this interaction is through epigenetics, which help grasp the neurobiological changes that occur in addiction and explain its relapsing-remitting nature. It is now known that every cell has a different way of expressing its phenotype, despite a universal DNA sequence. This is particularly true in the central nervous system where environmental factors influence this expression. Three major epigenetic processes have been found to participate in the perpetuation of addiction by changing the state of the chromatin and the degree of gene transcription: histone acetylation and methylation, DNA methylation, and noncoding RNAs. In the animal model literature, substantial evidence exists about the role of these epigenetic changes in the different phases of substance use disorders. This book chapter is a non-systematic literature review of the recent publications tackling the topic of epigenetics in addiction. Even though this evidence remains scarce and relatively poorly systematized, it is a promising foundation for future research of molecules that target specific brain regions and their functions to address core behavioral changes seen in addiction.

High-intensity binge drinking is associated with alterations in spontaneous neural oscillations in young adults

Heavy episodic alcohol consumption (also termed binge drinking) contributes to a wide range of health and cognitive deficits, but the associated brain-based indices are poorly understood. The current study used electroencephalography (EEG) to examine spontaneous neural oscillations in young adults as a function of quantity, frequency, and the pattern of their alcohol consumption. Sixty-one young adults (23.4 ± 3.4 years of age) were assigned to binge drinking (BD) and light drinking (LD) groups that were equated on gender, race/ethnic identity, age, educational background, and family history of alcoholism. EEG activity was recorded during eyes-open and eyes-closed resting conditions. Each participant's alpha peak frequency (APF) was used to calculate absolute power in individualized theta and alpha frequency bands, with a canonical frequency range used for beta. APF was slower by 0.7 Hz in BD, especially in individuals engaging in high-intensity drinking, but there were no changes in alpha power. BD also exhibited higher frontal theta and beta power than LD. Alpha slowing and increased theta power in BD remained after accounting for depression, anxiety, and personality characteristics, while elevated beta power covaried with sensation seeking. Furthermore, APF slowing and theta power correlated with various measures of alcohol consumption, including binge episodes and blackouts, but not with measures of working and episodic memory, cognitive flexibility, processing speed, or personality variables, suggesting that these physiological changes may be modulated by high-intensity alcohol intake. These results are consistent with studies of alcohol-use disorder (AUD) and support the hypothesis that binge drinking is a transitional stage toward alcohol dependence. The observed thalamocortical dysrhythmia may be indicative of an excitatory-inhibitory imbalance in BD and may potentially serve as an index of the progressive development of AUD, with a goal of informing possible interventions to minimize alcohol's deleterious effects on the brain.

Cortical Thickness in Adolescents with a Family History of Alcohol Use Disorder

BACKGROUND

Individuals with a family history (FH+) of alcohol use disorder (AUD) have a higher risk for developing an AUD than those with no family history (FH-) of AUD. In addition, FH+ individuals tend to perform worse on neuropsychological measures and show heightened impulsivity, which may be due to underlying differences in brain structure such as cortical thickness. The primary aim of this study was to investigate differences in cortical thickness in FH+ compared to FH- adolescents. Secondary aims were to (i) investigate differences in executive functioning and impulsivity, and (ii) examine associations between brain structure and behavior.

METHODS

Brain scans of 95 FH- and 93 FH+ subjects aged 13 to 18 were obtained using magnetic resonance imaging. FH+ subjects were required to have at least 1 biological parent with a history of an AUD. FH+ and FH- individuals had limited or no past alcohol use, thereby minimizing potential effects of alcohol. Subjects were evaluated on impulsivity and executive functioning tasks. Thicknesses of cortical lobes and subregions were analyzed using FreeSurfer. Regions showing group differences were examined for group-by-age interactions and correlations with neuropsychological and personality measures.

RESULTS

FH+ adolescents had thinner cortices in frontal and parietal lobes, notably in the medial orbitofrontal, lateral orbitofrontal, and superior parietal cortices. The difference in cortical thickness between family history groups was strongest among the youngest subjects. FH+ subjects were also more impulsive and had poorer performance on a spatial memory task.

CONCLUSIONS

These findings demonstrate frontal and parietal structural differences in FH+ adolescents that might underlie cognitive and behavioral characteristics associated with AUD risk.

Alcohol effects on the epigenome in the germline: Role in the inheritance of alcohol-related pathology

Excessive alcohol exposure has severe health consequences, and clinical and animal studies have demonstrated that disruptions in the epigenome of somatic cells, such as those in brain, are an important factor in the development of alcohol-related pathologies, such as alcohol-use disorders (AUDs) and fetal alcohol spectrum disorders (FASDs). It is also well known that alcohol-related health problems are passed down across generations in human populations, but the complete mechanisms for this phenomenon are currently unknown. Recent studies in animal models have suggested that epigenetic factors are also responsible for the transmission of alcohol-related pathologies across generations. Alcohol exposure has been shown to induce changes in the epigenome of sperm of exposed male animals, and these epimutations are inherited in the offspring. This paper reviews evidence for multigenerational and transgenerational epigenetic inheritance of alcohol-related pathology through the germline. We also review the literature on the epigenetic effects of alcohol exposure on somatic cells in brain, and its contribution to AUDs and FASDs. We note gaps in knowledge in this field, such as the lack of clinical studies in human populations and the lack of data on epigenetic inheritance via the female germline, and we suggest future research directions.

Glutamatergic transmission in the central nucleus of the amygdala is selectively altered in Marchigian Sardinian alcohol-preferring rats: Alcohol and CRF effects

The CRF system of the central nucleus of the amygdala (CeA) is important for the processing of anxiety, stress, and effects of acute and chronic ethanol. We previously reported that ethanol decreases evoked glutamate transmission in the CeA of Sprague Dawley rats and that ethanol dependence alters glutamate release in the CeA. Here, we examined the effects of ethanol, CRF and a CRF1 receptor antagonist on spontaneous and evoked glutamatergic transmission in CeA neurons from Wistar and Marchigian Sardinian Preferring (msP) rats, a rodent line genetically selected for excessive alcohol drinking and characterized by heightened activity of the CRF1 system. Basal spontaneous and evoked glutamate transmission in CeA neurons from msP rats was increased compared to Wistar rats. Ethanol had divergent effects, either increasing or decreasing spontaneous glutamate release in the CeA of Wistar rats. This bidirectional effect was retained in msP rats, but the magnitude of the ethanol-induced increase in glutamate release was significantly smaller. The inhibitory effect of ethanol on evoked glutamatergic transmission was similar in both strains. CRF also either increased or decreased spontaneous glutamate release in CeA neurons of Wistar rats, however, in msP rats CRF only increased glutamate release. The inhibitory effect of CRF on evoked glutamatergic transmission was also lost in neurons from msP rats. A CRF1 antagonist produced only minor effects on spontaneous glutamate transmission, which were consistent across strains, and no effects on evoked glutamate transmission. These results demonstrate that the genetically altered CRF system of msP rats results in alterations in spontaneous and stimulated glutamate signaling in the CeA that may contribute to both the anxiety and drinking behavioral phenotypes.

RhoGAP18B Isoforms Act on Distinct Rho-Family GTPases and Regulate Behavioral Responses to Alcohol via Cofilin

Responses to the effects of ethanol are highly conserved across organisms, with reduced responses to the sedating effects of ethanol being predictive of increased risk for human alcohol dependence. Previously, we described that regulators of actin dynamics, such as the Rho-family GTPases Rac1, Rho1, and Cdc42, alter Drosophila's sensitivity to ethanol-induced sedation. The GTPase activating protein RhoGAP18B also affects sensitivity to ethanol. To better understand how different RhoGAP18B isoforms affect ethanol sedation, we examined them for their effects on cell shape, GTP-loading of Rho-family GTPase, activation of the actin-severing cofilin, and actin filamentation. Our results suggest that the RhoGAP18B-PA isoform acts on Cdc42, while PC and PD act via Rac1 and Rho1 to activate cofilin. In vivo, a loss-of-function mutation in the cofilin-encoding gene twinstar leads to reduced ethanol-sensitivity and acts in concert with RhoGAP18B. Different RhoGAP18B isoforms, therefore, act on distinct subsets of Rho-family GTPases to modulate cofilin activity, actin dynamics, and ethanol-induced behaviors.

The central amygdala as an integrative hub for anxiety and alcohol use disorders

The central amygdala (CeA) plays a central role in physiologic and behavioral responses to fearful stimuli, stressful stimuli, and drug-related stimuli. The CeA receives dense inputs from cortical regions, is the major output region of the amygdala, is primarily GABAergic (inhibitory), and expresses high levels of prostress and antistress peptides. The CeA is also a constituent region of a conceptual macrostructure called the extended amygdala that is recruited during the transition to alcohol dependence. We discuss neurotransmission in the CeA as a potential integrative hub between anxiety disorders and alcohol use disorder, which are commonly co-occurring in humans. Imaging studies in humans and multidisciplinary work in animals collectively suggest that CeA structure and function are altered in individuals with anxiety disorders and alcohol use disorder, the end result of which may be disinhibition of downstream "effector" regions that regulate anxiety-related and alcohol-related behaviors.

Multigenerational and transgenerational inheritance of drug exposure: The effects of alcohol, opiates, cocaine, marijuana, and nicotine

Familial inheritance of drug abuse is composed of both genetic and environmental factors. Additionally, epigenetic transgenerational inheritance may provide a means by which parental drug use can influence several generations of offspring. Recent evidence suggests that parental drug exposure produces behavioral, biochemical, and neuroanatomical changes in future generations. The focus of this review is to discuss these multigenerational and transgenerational phenotypes in the offspring of animals exposed to drugs of abuse. Specifically, changes found following the administration of alcohol, opioids, cocaine, marijuana, and nicotine will be discussed. In addition, epigenetic modifications to the genome following administration of these drugs will be detailed as well as their potential for transmission to the next generation.

Ondansetron reduces naturalistic drinking in nontreatment-seeking alcohol-dependent individuals with the LL 5'-HTTLPR genotype: a laboratory study

BACKGROUND

One hypothesis suggests that the differential response to ondansetron- and serotonin-specific re-uptake inhibitors (SSRIs) may be due to a functional polymorphism of the 5'-HTTLPR promoter region in SLC6A4, the gene that codes for the serotonin transporter (5-HTT). The LL 5'-HTTLPR genotype is postulated to be specifically sensitive to the effects of ondansetron with SS/SL 5'-HTTLPR genotypes sensitive to SSRIs. This study tests this hypothesis by matching nontreatment-seeking alcohol-dependent (AD) individuals with LL genotype to ondansetron and SS/SL genotypes to the SSRI sertraline, and mismatching them assessing naturalistic and bar-laboratory alcohol drinking.

METHODS

Seventy-seven AD individuals were randomized to 1 of 2 counterbalanced arms to receive sertraline 200 mg/d or ondansetron 0.5 mg/d for 3 weeks followed by an alcohol self-administration experiment (ASAE) and then received placebo for 3 weeks followed by a second ASAE. Individuals then received the alternate drug for 3 weeks followed by a third ASAE. Drinks per drinking day (DDD with drinks in standard drinking units) for 7 days prior to each ASAE and milliliters consumed during each ASAE were the primary outcomes.

RESULTS

Fifty-five participants completed the study. The genotype × order interaction was significant, F(1, 47) = 8.42, p = 0.006, for DDD. Three analyses of covariance were conducted for DDD during the week before each ASAE. Ondansetron compared to sertraline resulted in a significant reduction in DDD during the week before the first, F(1, 47) = 7.64, p = 0.008, but not the third ASAE. There was no difference in milliliters consumed during each ASAE.

CONCLUSIONS

This study modestly supports the hypothesis that ondansetron may reduce DDD in AD individuals with the LL genotype as measured naturalistically. By contrast, there was no support that ondansetron reduces drinking during the ASAEs or that sertraline reduces alcohol use in individuals who have SS/SL genotypes. We provide limited support that ondansetron may reduce drinking in nontreatment-seeking individuals with the LL genotype.

Brain-derived neurotrophic factor (BDNF) Val66Met polymorphism and its implication in executive functions in adult offspring of alcohol-dependent probands

Impairment of executive functions (EFs) mediated by the prefrontal lobe is regarded as a cognitive endophenotype of alcohol dependence, being observed both in probands and in healthy offspring. Given its impact on the anatomy of the prefrontal cortex, the brain-derived neurotrophic factor (BDNF) Val66Met polymorphism may well be involved in this specific endophenotype. Forty-six healthy adult children of alcoholics (HACA) and 82 healthy controls (HC) took part in the study. All the participants were assessed with the Diagnostic Interview for Genetic Studies, and their family histories of alcohol and substance use were assessed with the Family Informant Schedule and Criteria. The Trail Making Test, Arithmetic Switching Task, Stroop Color-Word Test and Wisconsin Card Sorting Test were administered to assess EFs. An overall executive factor score was calculated using factorial analyses. Genotyping of the BDNF Val66Met polymorphism was performed using the TaqMan® allelic discrimination assay. HACA had significantly lower EFs performance than HC. Genetic analysis showed that BDNF genotype distributions were in Hardy-Weinberg equilibrium in the HACA and HC. Genotype and allele distributions did not differ significantly between the two groups. Participants with the Met allele performed significantly more poorly than participants with the Val allele, and a group by allele interaction was observed, the BDNF Met allele being associated with a poorer executive factor score in the HACA group. These results suggest that the BDNF Val66Met polymorphism may contribute to alcohol dependence vulnerability via lower EFs performance.

Role of a genetic polymorphism in the corticotropin-releasing factor receptor 1 gene in alcohol drinking and seeking behaviors of marchigian sardinian alcohol-preferring rats

Marchigian Sardinian alcohol-preferring (msP) rats exhibit innate preference for alcohol, are highly sensitive to stress and stress-induced alcohol seeking. Genetic analysis showed that over-expression of the corticotropin-releasing factor (CRF) system of msP rats is correlated with the presence of two single nucleotide polymorphisms (SNPs) occurring in the promoter region (position -1836 and -2097) of the CRF1 receptor (CRF1-R) gene. Here we examined whether these point mutations were associated to the innate alcohol preference, stress-induced drinking, and seeking. We have recently re-derived the msP rats to obtain two distinct lines carrying the wild type (GG) and the point mutations (AA), respectively. The phenotypic characteristics of these two lines were compared with those of unselected Wistar rats. Both AA and GG rats showed similar patterns of voluntary alcohol intake and preference. Similarly, the pharmacological stressor yohimbine (0.0, 0.625, 1.25, and 2.5 mg/kg) elicited increased operant alcohol self-administration under fixed and progressive ratio reinforcement schedules in all three lines. Following extinction, yohimbine (0.0, 0.625, 1.25, and 2.5 mg/kg) significantly reinstated alcohol seeking in the three groups. However, at the highest dose this effect was no longer evident in AA rats. Treatment with the CRF1-R antagonist antalarmin (0, 5, 10, and 20 mg/kg) significantly reduced alcohol-reinforced lever pressing in the AA line (10 and 20 mg/kg) while a weaker or no effect was observed in the Wistar and the GG group, respectively. Finally, antalarmin significantly reduced yohimbine-induced increase in alcohol drinking in all three groups. In conclusion, these specific SNPs in the CRF1-R gene do not seem to play a primary role in the expression of the msP excessive drinking phenotype or stress-induced drinking but may be associated with a decreased threshold for stress-induced alcohol seeking and an increased sensitivity to the effects of pharmacological blockade of CRF1-R on alcohol drinking.

Enhanced GABAergic transmission in the central nucleus of the amygdala of genetically selected Marchigian Sardinian rats: alcohol and CRF effects

The GABAergic system in the central amygdala (CeA) plays a major role in ethanol dependence and the anxiogenic-like response to ethanol withdrawal. Alcohol dependence is associated with increased corticotropin releasing factor (CRF) influence on CeA GABA release and CRF type 1 receptor (CRF(1)) antagonists prevent the excessive alcohol consumption associated with dependence. Genetically selected Marchigian Sardinian (msP) rats have an overactive extrahypothalamic CRF(1) system, are highly sensitive to stress, and display an innate preference for alcohol. The present study examined differences in CeA GABAergic transmission and the effects of ethanol, CRF and a CRF(1) antagonist in msP, Sprague Dawley, and Wistar rats using an electrophysiological approach. We found no significant differences in membrane properties or mean amplitude of evoked GABA(A)-inhibitory postsynaptic potentials (IPSPs). However, paired-pulse facilitation (PPF) ratios of evoked IPSPs were significantly lower and spontaneous miniature inhibitory postsynaptic current (mIPSC) frequencies were higher in msP rats, suggesting increased CeA GABA release in msP as compared to Sprague Dawley and Wistar rats. The sensitivity of spontaneous GABAergic transmission to ethanol (44 mM), CRF (200 nM) and CRF(1) antagonist (R121919, 1 μM) was comparable in msP, Sprague Dawley, and Wistar rats. However, a history of ethanol drinking significantly increased the baseline mIPSC frequency and decreased the effects of a CRF(1) antagonist in msP rats, suggesting increased GABA release and decreased CRF(1) sensitivity. These results provide electrophysiological evidence that msP rats display distinct CeA GABAergic activity as compared to Sprague Dawley and Wistar rats. The elevated GABAergic transmission observed in naïve msP rats is consistent with the neuroadaptations reported in Sprague Dawley rats after the development of ethanol dependence.

Custom genotyping for substance addiction susceptibility genes in Jordanians of Arab descent

Background

Both environmental and genetic factors contribute to individual susceptibility to initiation of substance use and vulnerability to addiction. Determining genetic risk factors can make an important contribution to understanding the processes leading to addiction. In order to identify gene(s) and mechanisms associated with substance addiction, a custom platform array search for a genetic association in a case/control of homogenous Jordanian Arab population was undertaken. Patients meeting the DSM-VI criteria for substance dependence (n = 220) and entering eight week treatment program at two Jordanian Drug Rehabilitation Centres were genotyped. In addition, 240 healthy controls were also genotyped. The sequenom MassARRAY system (iPLEX GOLD) was used to genotype 49 single nucleotide polymorphisms (SNPs) within 8 genes (DRD1, DRD2, DRD3, DRD4, DRD5, BDNF, SLC6A3 and COMT).

Results

This study revealed six new associations involving SNPs within DRD2 gene on chromosome 11. These six SNPs within the DRD2 were found to be most strongly associated with substance addiction in the Jordanian Arabic sample. The strongest statistical evidence for these new association signals were from rs1799732 in the C/−C promoter and rs1125394 in A/G intron 1 regions of DRD2, with the overall estimate of effects returning an odds ratio of 3.37 (χ2 (2, N = 460) = 21, p-value = 0.000026) and 1.78 (χ2 (2, N = 460) = 8, p-value = 0.001), respectively. It has been suggested that DRD2, dopamine receptor D2, plays an important role in dopamine secretion and the signal pathways of dopaminergic reward and drug addiction.

Conclusion

This study is the first to show a genetic link to substance addiction in a Jordanian population of Arab descent. These findings may contribute to our understanding of drug addiction mechanisms in Middle Eastern populations and how to manage or dictate therapy for individuals. Comparative analysis with different ethnic groups could assist further improving our understanding of these mechanisms.

Association of the 5-HTT gene-linked promoter region (5-HTTLPR) polymorphism with psychiatric disorders: review of psychopathology and pharmacotherapy

Serotonin (5-HT) regulates important biological and psychological processes including mood, and may be associated with the development of several psychiatric disorders. An association between psychopathology and genes that regulate 5-HT neurotransmission is a robust area of research. Identification of the genes responsible for the predisposition, development, and pharmacological response of various psychiatric disorders is crucial to the advancement of our understanding of their underlying neurobiology. This review highlights research investigating 5-HT transporter (5-HTTLPR) polymorphism, because studies investigating the impact of the 5-HTTLPR polymorphism have demonstrated significant associations with many psychiatric disorders. Decreased transcriptional activity of the S allele ("risk allele") may be associated with a heightened amygdala response leading to anxiety-related personality traits, major depressive disorder, suicide attempts, and bipolar disorder. By contrast, increased transcriptional activity of the L allele is considered protective for depression but is also associated with completed suicide, nicotine dependence, and attention deficit hyperactivity disorder. For some disorders, such as post-traumatic stress disorder and major depressive disorder, the research suggests that treatment response may vary by allele (such as an enhanced response to serotonin specific reuptake inhibitors in patients with major depressive disorder and post-traumatic stress disorder with L alleles), and for alcohol dependence, the association and treatment for S or L alleles may vary with alcoholic subtype. While some studies suggest that 5-HTTLPR polymorphism can moderate the response to pharmacotherapy, the association between 5-HTTLPR alleles and therapeutic outcomes is inconsistent. The discovery of triallelic 5-HTTLPR alleles (L(A)/L(G)/S) may help to explain some of the conflicting results of many past association studies, while concurrently providing more meaningful data in the future. Studies assessing 5-HTTLPR as the solitary genetic factor contributing to the etiology of psychiatric disorders continue to face the challenges of statistically small effect sizes and limited replication.

Drugs and addiction: an introduction to epigenetics

Addiction is a debilitating psychiatric disorder, with a complex aetiology involving the interaction of inherited predispositions and environmental factors. Emerging evidence suggests that epigenetic alterations to the genome, including DNA methylation and histone modifications, are important mechanisms underlying addiction and the neurobiological response to addictive substances. In this review, we introduce the reader to epigenetic mechanisms and describe a potential role for dynamic epigenetic changes in mediating addictive behaviours via long-lasting changes in gene expression. We summarize recent findings from both molecular and behavioural experiments elucidating the role of epigenetic changes in mediating the addictive potential of various drugs of abuse, including cocaine, amphetamine and alcohol. The implications of these findings for molecular studies of addiction and the future development of novel therapeutic interventions are also discussed.

Children of men with alcohol dependence: Psychopathology, neurodevelopment and family environment

BACKGROUND

Children of people with alcohol dependence (COAs) are at high risk for behavioral and cognitive problems.

AIM

Aim of this study was to compare the nature and extent of these problems in children of men with and without alcohol dependence.

MATERIALS AND METHODS

32 children (17 in study group and 15 controls) were evaluated for psychopathology, neurodevelopment, cognitive functioning and family environment. Tools used were: Socio-demographic data sheet, Malin's Intelligence Scale for Indian Children (MISIC), Child Behavior Checklist, Trail Making Test, Neurodevelopment Scale and the Family Environment Scale.

RESULTS

Children of men with alcohol dependence had higher externalizing than internalizing scores. Children of alcohol-dependent fathers had higher scores on the neurodevelopment scale and lower scores on the performance scale of the MISIC than the children in control group. These children also made more errors on the Trail Making Test. The family environment of COAs was characterized by lack of independence for its members, greater perceived control and lack of adequate cultural and intellectual activities.

CONCLUSION

Our findings suggest that children of men with alcohol dependence have difficulties with frontal lobe functions and neurodevelopmental tasks. There are also difficulties in the family, which are related to alcohol consumption by the father.

Neural circuitry associated with risk for alcohol use disorders

The core features of risk for alcohol use disorders (AUD), including behavioral disinhibition, affective dysregulation, and executive dysfunction, map onto distinct neural circuits that have been found to be abnormal in the offspring of alcohol dependent individuals. Components of the cerebellothalamocortical system and the extended limbic network may provide the underpinnings for the behavioral and emotional dysfunction observed in individuals at heightened risk for AUD. In addition, abnormalities in these structures appear to be altered in individuals with the predisposition for other psychiatric conditions that may share a similar genetic diathesis. This review proposes several neurobehavioral mechanisms of genetic vulnerability that may account for phenotypic characteristics in individuals at risk for AUD.

A within-group design of nontreatment seeking 5-HTTLPR genotyped alcohol-dependent subjects receiving ondansetron and sertraline

BACKGROUND

Serotonergic mechanisms are associated with the development of alcohol dependence (AD), however, studies evaluating serotonergic medications have produced conflicting results. One hypothesis suggests that differential response may be due to a functional polymorphism of the 5-HTTLPR promoter region of the serotonin re-uptake transporter (5-HTT). The L/L genotype is postulated to be associated with early onset alcoholism and the S/S or S/L genotypes associated with late onset alcoholism. The aim of this study was to match and mismatch L/L, S/S, or S/L genotypes with administration of ondansetron and sertraline.

METHODS

Fifteen nontreatment seeking alcohol-dependent individuals were randomized to 1 of 2 counterbalanced arms to receive either 200 mg/d of sertraline or ondansetron 0.5 mg/d for 3 weeks followed by an alcohol self-administration experiment (ASAE), then received placebo for 3 weeks followed by a second ASAE. Participants then received the alternate drug for 2 weeks followed by a third ASAE.

RESULTS

At the first ASAE compared to sertraline, ondansetron significantly improved drinking outcomes for the L/L genotype for the ASAE volume consumed (100% reduction based on between-subjects comparison, t = 2.35), and for drinks per drinking day during the 7 days prior to the ASAE (79% reduction and t = 4.34). Compared with ondansetron for S/S or S/L genotypes, outcomes at ASAE 1 for sertraline and S/S or S/L genotypes are opposite than hypothesized. Overall, subjects reduced drinking across their participation in the trial, as there appears to be an order effect.

CONCLUSION

This study suggests that ondansetron may reduce alcohol consumption in alcohol-dependent individuals who have the L/L genotype as measured naturalistically and during the ASAE. By contrast there was no support that sertraline reduces alcohol use in individuals who have S/S or S/L genotypes. Evidence in the literature suggests that AD in some individuals may be influenced by a gene by socio-environmental interaction making pharmacological treatment with serotonergic drugs complex. Research must consider that typologies may predict successful treatment of AD in future trials.

Review. Genetics of addictions: strategies for addressing heterogeneity and polygenicity of substance use disorders

Addictions are common psychiatric disorders that exert high cost to the individual and to society. Addictions are a result of the interplay of multiple genetic and environmental factors. They are characterized by phenotypic and genetic heterogeneity as well as polygenicity, implying a contribution of different neurobiological mechanisms to the clinical diagnosis. Therefore, treatments for most substance use disorders are often only partially effective, with a substantial proportion of patients failing to respond. To address heterogeneity and polygenicity, strategies have been developed to identify more homogeneous subgroups of patients and to characterize genes contributing to their phenotype. These include genetic linkage and association studies as well as functional genetic analysis using endophenotypes and animal behavioural experimentation. Applying these strategies in a translational context aims at improving therapeutic response by the identification of subgroups of addiction patients for individualized, targeted treatment strategies. This article aims to discuss strategies addressing heterogeneity and polygenicity of substance use disorders by presenting results of recent research on genetic and environmental components of addiction. It will also introduce the European IMAGEN study that aims to integrate methodical approaches discussed in order to identify the genetic and neurobiological basis of behavioural traits relevant to the development of addictions.

Cognitive deficits in children of alcoholics: At risk before the first sip!

BACKGROUND

High family loading for alcoholism, early onset of alcohol use and childhood disinhibitory behaviors, persisting into adulthood, increase the susceptibility to alcoholism. At the psychophysiology level, reduced amplitude of the P300 component of the Evoked Response Potential is associated with externalizing psychopathology in children. Children of alcoholics have reduced P300 amplitudes. Preliminary data suggests a developmental lag phenomenon in the maturation of the P300.

AIMS

The study compares the amplitude and topography of the P300 generated in response to a visual task, between subjects at high risk (HR) and those at low risk (LR) for alcoholism and its relation to externalizing behaviors.

RESULTS

HR subjects have lower P300 amplitudes over frontal brain areas. Differences are greater in young, tending to converge with increasing age. There is a strong association between this reduced brain activation and an excess of externalizing behaviors in HR individuals.

CONCLUSION

A maturational lag in brain development causing central nervous system disinhibition and externalizing behaviors may underlie the susceptibility to alcoholism.

Gray matter volume abnormalities and externalizing symptoms in subjects at high risk for alcohol dependence

Reduced right amygdala volumes have been reported in young, alcohol-naïve subjects at high risk (HR) for alcohol dependence. The differences in brain morphometry have been associated with an excess of externalizing behaviors in these subjects. This may reflect a neurobiological vulnerability to alcohol dependence. Existing Magnetic Resonance Imaging (MRI) studies on these subjects have examined only a few, pre-selected brain regions using the manual regions of interest (ROI) approach. MRI of HR subjects (n = 20) and age, sex, and handedness-matched low-risk (LR) subjects (n = 21) were analyzed using optimized voxel-based morphometry and ROI approach. The externalizing symptoms of these subjects and their fathers were measured using the Semi-Structured Assessment for the Genetics of Alcoholism. HR subjects had significantly smaller volumes of superior frontal, cingulate and parahippocampal gyri, amygdala, thalamus and cerebellum. These gray matter volumes correlated negatively with externalizing symptoms scores. Subjects at HR for alcoholism have reduced volumes of critical areas of brain gray matter, which are associated with increased externalizing symptoms. These represent key endophenotypes of alcoholism.

Distinct behavioral responses to ethanol are regulated by alternate RhoGAP18B isoforms

In most organisms, low ethanol doses induce increased activity, while high doses are sedating. To investigate the underlying mechanisms, we isolated Drosophila mutants with altered ethanol responsiveness. Mutations in white rabbit (whir), disrupting RhoGAP18B, are strongly resistant to the sedating effects of ethanol. This resistance can be suppressed by reducing the levels of Rho1 or Rac, implicating these GTPases in the behavioral response to ethanol. Indeed, expression of constitutively active forms of Rho1 or Rac1 in adult flies results in ethanol resistance similar to that observed in whir mutants. The whir locus produces several transcripts, RA-RD, which are predicted to encode three distinct RhoGAPs that share only the GAP domain. The RC transcript mediates the sedating effects of ethanol, while the RA transcript regulates its stimulant effects. Thus, distinct RhoGAPs, encoded by the same gene, regulate different manifestations of acute ethanol intoxication.

Driving actin dynamics under the influence of alcohol

The actin cytoskeleton plays a pivotal role in regulating neuronal development and activity, and dysregulation of actin dynamics has been linked to impaired cognitive function. In this issue of Cell Rothenfluh et al. (2006) , and Offenhäuser et al. (2006) show that actin dynamics can also affect the cellular and behavioral responses of flies and mice to alcohol.

Addiction to opioids in chronic pain patients: a literature review

Opioids have proven very useful for treatment of acute pain and cancer pain, and in the developed countries opioids are increasingly used for treatment of chronic non-malignant pain patients as well. This literature review aims at giving an overview of definitions, mechanisms, diagnostic criteria, incidence and prevalence of addiction in opioid treated pain patients, screening tools for assessing opioid addiction in chronic pain patients and recommendations regarding addiction problems in national and international guidelines for opioid treatment in cancer patients and chronic non-malignant pain patients. The review indicates that the prevalence of addiction varied from 0% up to 50% in chronic non-malignant pain patients, and from 0% to 7.7% in cancer patients depending of the subpopulation studied and the criteria used. The risk of addiction has to be considered when initiating long-term opioid treatment as addiction may result in poor pain control. Several screening tools were identified, but only a few were thoroughly validated with respect to validity and reliability. Most of the identified guidelines mention addiction as a potential problem. The guidelines in cancer pain management are concerned with the fact that pain may be under treated because of fear of addiction, and the guidelines in management of non-malignant pain patients include warnings of addiction. According to the literature, it seems appropriate and necessary to be aware of the problems associated with addiction during long-term opioid treatment, and specialised treatment facilities for pain management or addiction medicine should be consulted in these cases.

Alcohol use in families: a 15-year prospective follow-up study

AIMS

To study the prevalence of adolescents' problematic alcohol use and its parental predictors: drinking habits and socio-demographic factors.

DESIGN, SETTING, PARTICIPANTS AND MEASUREMENTS

Questionnaires were sent regularly to the same Finnish families (n = 1132) from the onset of pregnancy (in 1986-87) to the child's age of 15 years (in 2001-02) (n = 1028). There was a total of three follow-up points.

FINDINGS

At 15 years of age, 83% of girls and 79% of boys had used alcohol; 18% of boys and 14% of girls had been drunk more than once a month. The child's permanent separation from at least one biological parent was the strongest socio-demographic predictor of adolescents drinking. Fathers' present heavy drinking and parental early drinking were the best predictors of their children's problematic alcohol use at the age of 15.

CONCLUSIONS

Knowledge of fathers' alcohol use and its time of onset may be used to determine children who are at added risk of problematic alcohol use later in life. Special guidance, support and treatment can be targeted to these families.

Genetic association of the human corticotropin releasing hormone receptor 1 (CRHR1) with binge drinking and alcohol intake patterns in two independent samples

To investigate the role of the corticotropin releasing hormone receptor 1 (CRHR1) in patterns of human alcohol drinking and its potential contribution to alcohol dependence, we analysed two independent samples: a sample of adolescents, which consisted of individuals from the 'Mannheim Study of Risk Children' (MARC), who had little previous exposure to alcohol, and a sample of alcohol-dependent adults, who met DSM-IV criteria of alcohol dependence. Following determination of allelic frequencies of 14 polymorphisms of the CRHR1 gene, two haplotype tagging (ht)SNPs discriminating between haplotypes with a frequency of > or =0.7% were identified. Both samples were genotyped and systematically examined for association with the htSNPs of CRHR1. In the adolescent sample, significant group differences between genotypes were observed in binge drinking, lifetime prevalence of alcohol intake and lifetime prevalence of drunkenness. The sample of adult alcohol-dependent patients showed association of CRHR1 with high amount of drinking. This is the first time that an association of CRHR1 with specific patterns of alcohol consumption has been reported. Our findings support results from animal models, suggesting an importance of CRHR1 in integrating gene-environment effects in alcohol use disorders.

Substance use and other psychiatric disorders in first-degree relatives of opioid-dependent males: a case-controlled study from India

To assess the prevalence of substance use and other psychiatric disorders in first-degree relatives of males with opioid dependence compared to normal controls.

DESIGN

Observational, case-control study using family history method.

SETTING

A drug addiction treatment centre in northern India.

PARTICIPANTS

First-degree relatives of 100 male probands with opioid dependence and no comorbidity (n=493) and those of 50 matched probands from normal population (n=254). Measurement Family interview of probands and family members, using the Family Interview for Genetic Studies. The main outcome measure was relative risk (expressed as odds ratio after controlling for confounding variables using logistic regression) of familial aggregation of psychiatric and substance use disorders.

FINDINGS

First-degree relatives of opioid-dependent males were more likely to have a psychiatric disorder than those of normal controls [adjusted odds ratio (OR) 4.47; 95% confidence interval (CI) 1.97-10.11; P<0.001], especially for opioid use disorders in the brothers (adjusted OR 6.55; 95% CI 1.44-29.88; P=0.015) and for alcohol use disorders in the fathers of the probands (adjusted OR 5.64; 95% CI 2.39-13.24; P<0.001). Other disorders (major depression, chronic psychosis and obsessive compulsive disorder) did not have significant aggregation in the first-degree relatives of opioid-dependent subjects.

CONCLUSIONS

This study provides further evidence for the higher rates of alcohol and opioid dependence in first-degree relatives of opioid-dependent patients. The exact pattern of this familial aggregation may be influenced by the gender of the relatives and their relation to the proband.

Identifikation von Risikogenen für Alkoholabhängigkeit

Alcohol dependence is one of the most common addictive diseases and known to be in part genetically transmitted, based on an oligogenic background in which each gene involved contributes only little to the resulting phenotype. Besides influencing other signal transduction mechanisms, alcohol specifically inhibits the NMDA signaling cascade, which mediates the excitatory effects of glutamate in the brain. Target molecules, sensitive to ethanol, include the NMDA receptors as well as downstream molecules of the glutamatergic system, glutamate transporters, and associated regulatory proteins. Adaptive processes of the glutamatergic system during chronic alcohol consumption may play a major role for later development of reward symptoms. Candidate gene studies, including association studies and animal models, are powerful and sensitive for detecting oligogenic effects and thus important to alcoholism research.

Association study of brain-derived neurotrophic factor gene polymorphism and alcoholism

OBJECTIVE

Brain-derived neurotrophic factor (BDNF) influences dopamine and serotonin neurotransmitters that are heavily linked to addiction. A quantitative trait loci study indicated that genes localized to 11p13, where the BDNF gene is mapped (11p13-15), increase the risk for severe alcohol withdrawal. Moreover, a recent study using a pooled-sample microarray suggested that the BDNF gene locus was included in the loci that were shown to be associated with drug abuse. These lines of evidence suggested that BDNF might play some role in the development of or vulnerability to alcoholism and/or clinical characteristics of alcoholic individuals.

METHODS

The alcoholic subjects consisted of 377 male Japanese patients. A structured interview was used to obtain social background, drinking history, history of violence while intoxicated, history of alcohol withdrawal, and family history of alcoholism. The control group consisted of 336 nonalcoholic male subjects. Genotyping of the G196A polymorphism of the BDNF gene was done by polymerase chain reaction (PCR)-restriction fragment length polymorphism method.

RESULTS

Genotype and allele distributions of the BDNF gene polymorphism did not differ significantly between alcoholic and control subjects. However, comparing clinical characteristics across G196A genotypes, we found that alcoholic subjects with violent tendencies and a history of delirium tremens had a significantly higher frequency of AA genotypes and A allele frequencies than those without them. Moreover, alcoholic subjects with the A allele had earlier onset of the disease than those without it.

CONCLUSIONS

These results indicate that BDNF gene polymorphism might modify phenotypes of alcoholism.

Typologies of drug dependence: comparative validity of a multivariate and four univariate models

Data from a longitudinal cohort study were used to directly compare the concurrent and predictive validity of four univariate typologic approaches with a multivariate approach in subtyping drug dependence. The four univariate typologies were based upon: (a) age-of-onset of drug abuse/dependence, (b) presence of drug abuse in first-degree relatives, (c) presence of antisocial personality disorder, and (d) sex. The multivariate typologic approach was based on indices of vulnerability, chronicity, consequences, and psychopathology, yielding the Type A/B dichotomy first demonstrated in alcohol dependence. Subtypes generated from the univariate typologies were then each compared with the multivariate typology on measures of concurrent and predictive validity, and the strength of association was compared statistically. There was evidence of significantly greater concurrent validity of the Type A/B typology compared with the univariate typologies across all the domains of validation (risk, substance use, psychopathology, personality, and overall functioning). The multivariate typology also fared better than the univariate ones in all three domains on which predictive validity was evaluated: substance use, psychopathology, and overall functioning, as well as the degree of change in several composite scores (drug, medical, legal, and psychiatric) and the global psychiatric symptom index. This direct method of comparison seemed to demonstrate the superior validity of the multivariate cluster-analytic approach over the univariate approaches to classifying subjects with drug dependence.

Manipulations during the second, but not the first, week of life increase susceptibility to cocaine self-administration in female rats

We compared the effects of manipulations during week 1 vs week 2 of life on the propensity to self-administer cocaine. Pups received daily subcutaneous saline injections, were handled briefly, or remained undisturbed during their respective treatment periods. Animals handled during the second week of life exhibited increased locomotor response to novelty when tested on postnatal day (PND) 48, compared to all other groups. Rats were implanted with jugular catheters on PND 70 and then given the opportunity to self-administer (0.125 mg/kg/infusion) cocaine for 5 consecutive days (1 h sessions). The dose was then raised to 0.25 mg/kg/infusion for 5 days and to 0.5 mg/kg/infusion for the final 5 days of testing. Only animals manipulated during the second week of life acquired drug-taking behavior. These effects were both stimulus- and gender-specific. Females handled during the second week of life acquired cocaine self-administration (SA) at the lowest dose, and females injected during the second week of life acquired at the intermediate dose. Males injected during the second week of life showed a similar, but more variable, drug-taking pattern. There were no group differences in serum corticosterone response to novelty, although relative to undisturbed animals and those manipulated in the first week of life, female animals manipulated during the second week of life had lower basal expression of hippocampal glucocorticoid receptor mRNA in adulthood. We conclude that the second week of life in the rodent is a sensitive period during which manipulations result in a more vulnerable phenotype for the acquisition of cocaine SA.

Linkage disequilibrium and haplotype analysis between serotonin receptor 1B gene variations and subtypes of alcoholism

A number of studies have reported a possible association between serotonergic pathway genes and alcoholism. A silent polymorphism (G to C substitution) in the gene encoding the autoreceptor 5-HT1B was linked to antisocial alcoholism in Finnish and an American Indian populations [Lappalainen et al., 1998: Arch Gen Psychiatry 55:989-994]. Several other polymorphisms of this gene have been investigated for their association with neuropsychiatric disorders. In the present study, a sample of 133 alcoholics without and 39 alcoholics with medical complications, and 88 normal controls was screened for three single nucleotide polymorphisms (SNPs), G861C, G261T, and C129T, in the 5-HT1B gene. The goal was to investigate their association with the disease, to measure the strength of linkage disequilibrium (LD) between the SNPs, and to compare haplotype frequencies between alcoholic groups and normal controls. Data was also analyzed on the basis of Type I (n = 47) and Type II (n = 85) alcoholism. There was no significant difference in the allele frequencies or the genotype distribution between any alcoholic groups, alcoholic subgroups, and controls for any polymorphism. G861C and C129T polymorphisms were in complete LD. The pattern of distribution of haplotypes was similar in patients and controls. It is concluded that these SNPs are not playing any direct role in the development of susceptibility to alcoholism in our patient sample.

Alcohol addiction research: from animal models to clinics

Addictive behaviour evolves only on the basis of voluntary drug intake. As a consequence, when designing an animal model that covers several aspects of alcohol dependence and other alcohol related-diseases a necessary precondition is that the animal has voluntary access to alcohol. Animal models on voluntary alcohol consumption have a long-standing tradition in biomedical research on alcoholism. However, preference studies allow only limited conclusions regarding alcohol dependence and addictive behaviour. Therefore, new animal models have been developed that mimic different aspects of human alcohol dependence such as craving, relapse and loss of control over drinking. These models include the reinstatement model, the alcohol deprivation model and the point-of-no-return model. These models have now been pharmacologically validated using anti-craving compounds that are used clinically for treating alcoholics. In conclusion, there appears to be a good correspondence between the events that induce relapse and loss of control over alcohol-taking behaviour in laboratory animals and those that provoke relapse and loss of control in humans.

Drosophila melanogaster, a genetic model system for alcohol research

In its natural environment, which consists of fermenting plant materials, the fruit fly Drosophila melanogaster encounters high levels of ethanol. Flies are well equipped to deal with the toxic effects of ethanol; they use it as an energy source and for lipid biosynthesis. The primary ethanol-metabolizing pathway in flies involves the enzymes alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH); their role in adaptation to ethanol-rich environments has been studied extensively. The similarity between Drosophila and mammals is not restricted to the manner in which they metabolize ethanol; behaviors elicited by ethanol exposure are also remarkably similar in these organisms. Flies show signs of acute intoxication, which range from locomotor stimulation at low doses to complete sedation at higher doses, they develop tolerance upon intermittent ethanol exposure, and they appear to like ethanol, showing preference for ethanol-containing media. Molecular genetic analysis of ethanol-induced behaviors in Drosophila, while still in its early stages, has already revealed some surprising parallels with mammals. The availability of powerful tools for genetic manipulation in Drosophila, together with the high degree of conservation at the genomic level, make Drosophila a promising model organism to study the mechanism by which ethanol regulates behavior and the mechanisms underlying the organism's adaptation to long-term ethanol exposure.

Definitions related to the medical use of opioids: evolution towards universal agreement

Misunderstandings regarding the nature and occurrence of addiction have historically been barriers to the appropriate treatment of pain and have stigmatized the medical use of opioids. This article reviews the evolution of nomenclature related to addiction, presents current scientific understanding of addiction that may help shape universally acceptable terminology, and discusses an integrated effort of pain and addiction professionals to reach consensus on addiction-related terms. The article suggests key principles that may clarify terminology including: clear differentiation of the concepts of addiction and physical dependence, conceptualization of addiction as a multidimensional disease, and use of a label for the phenomenon of addiction that does not include the ambiguous term "dependence." More universal agreement on terminology related to addiction is expected to improve the treatment of both pain and addictive disorders; improve communication between health care providers, regulators, and enforcement agencies; and reduce health care and other societal costs.

Transcription profiling reveals mitochondrial, ubiquitin and signaling systems abnormalities in postmortem brains from subjects with a history of alcohol abuse or dependence

Alcohol abuse is a common human disorder with high rate of comorbidity with other psychiatric disorders. To identify candidate mechanisms for alcohol abuse, the expression of 12,626 genes was measured in postmortem temporal cortex from 11 subjects with a history of alcohol abuse or dependence, with or without other psychiatric diagnoses and compared pairwise with the expression in 11 nonalcoholic subjects matched for the other psychiatric diagnoses and demographics. Genes were defined to have altered expression in alcohol abuse if: 1) the gene showed decreased expression in at least 10 of 11 subjects with alcohol abuse, or showed increased expression in at least 10 of 11 subjects with this diagnosis compared to matched non-abusers (P < 0.007, chi(2)test); or 2) the difference in the mean abuser/non-abuser ratio for the gene from value of 1.0 was significant at P < 0.05 (one sample t-test). In subjects with a history of alcohol abuse or dependence, 163 genes were changed significantly. The most abundant and consistent changes were in gene families encoding mitochondrial proteins, the ubiquitin system, and signal transduction. These alterations indicate disturbances in energy metabolism and multiple signaling mechanisms in the temporal cortex of subjects with a history of alcohol abuse or dependence. We hypothesize that these mechanisms may be related to alcohol abuse traits or long-term effects of alcohol.

Regional CNS densities of serotonin 1A and dopamine D2 receptors in periadolescent alcohol-preferring P and alcohol-nonpreferring NP rat pups

The objective of the present study was to use quantitative autoradiography to determine binding densities of serotonin(1A) (5-HT(1A)) and dopamine (DA) D(2) receptors in alcohol-naive periadolescent P and NP rat pups. P (n=8) and NP (n=7) rat pups, 25 days of age, from different litters were used. Coronal brain sections were incubated with 2 nM [3H]8-OH-DPAT or 20 nM [3H]sulpiride for 5-HT(1A) or D(2) binding, respectively. Approximately 15-40% higher densities of [3H]8-OH-DPAT binding were observed in the anterior cortical regions of the periadolescent P rat compared with NP rat pups. Similar differences were also observed in posterior cortical regions with P rats having 25-40% higher [3H]8-OH-DPAT binding than NP rats. [3H]8-OH-DPAT binding was approximately 10-20% higher in posterior hippocampal regions of the P rat pups compared with the NP line. [3H]sulpiride binding was significantly different only in the ventral tegmental area (VTA), where binding was approximately 20% lower in the periadolescent P rats compared with the NP rat pups. Overall, these results are very similar to findings observed in adult alcohol-naive P and NP rats, and suggest that the innate differences in the neural systems implicated in high alcohol drinking behaviors may already be established in the periadolescent animal.

Neurodevelopmental liabilities in alcohol dependence: central serotonin and dopamine dysfunction

Alcoholism is a complex disorder with symptoms ranging from abuse to dependence, often comorbid with depression, antisocial personality, or anxiety. Neurodevelopmental causes of the disorder are unknown but inferences are possible from current knowledge. Neurobiological studies implicate multiple brain changes, which may be characterized as premorbid or morbid. These studies have also examined specific aspects of the alcohol dependence syndrome, including alcohol reinforcement and craving. Here, we review the evidence for vulnerability factors in alcohol dependence, with an emphasis on central serotonin (5-HT) and dopamine (DA). Serotonin dysfunction likely contributes to the development of alcoholism since studies of alcohol-preferring rodents show decreased 5-HT function on many measures. We have shown that serotonin-enhancing drugs reduce consumption and craving in mild to moderate alcoholics, yet similar studies in severely dependent individuals remain inconclusive. Studies indicate that serotonin dysfunction may contribute to the development of dependence via impaired impulse control and/or mood regulation. The mesocorticolimbic dopamine pathway represents another important pathophysiological target in alcoholism. Differences in D(2) receptor density, dopamine sensitivity, and gene expression have been linked to consumption, reinforcement, craving, and relapse. However, while DA agonists reduce self-administration in animals, we found no effect in humans with long-acting bromocriptine, a D(2) agonist. Dopamine may contribute differentially to the development of dependence via its effects on alcohol wanting, reinforcement, and reward memory. Although animal experiments show consistent roles for serotonin and dopamine in alcohol dependence, human studies are not always concordant. Such discrepancies highlight the complexity of dependence-related behaviors in humans and of identifying vulnerabilities to alcoholism.

The HTR1B 861G>C receptor polymorphism among patients suffering from alcoholism, major depression, anxiety disorders and narcolepsy

The HTR1B receptor gene has been linked to antisocial alcoholism in a Finnish population and an American Indian tribe [Lappalainen et al. , Arch. Gen. Psychiatry, 55 (1998) 989]. Using a candidate gene approach, we genotyped 209 patients with alcoholism, 108 patients with major depression, 32 patients with panic disorder, 50 patients with generalized anxiety disorder, 58 patients with narcolepsy and 74 healthy volunteers for the HTR1B 861G>C polymorphism. There was a higher frequency of the HTR1B 861G alleles among the alcohol-dependent patients as compared to the control subjects (chi(2)=4.02, d.f.=2, P=0.04). However, the association resulted from higher frequencies of the opposite alleles (HTR1B 861G), as originally reported by Lappalainen et al. (1998). Although the association in our study might be due to a type I error, the higher degree of HTR1B allele sharing within both populations could also argue for another alcoholism-relevant gene within the proximity of the HTR1B gene on human chromosome 6.